From my understanding PRK is allowable, however there is a certain amount of time that must pass after the surgery, and a waiver depending on what program you are going for. The Coast Guard, being a smaller services, seems to be a bit picky about waivers. Just a disclaimer, I do not work for the Coast Guard. I went through the enlistment process with them recently, and dropped for a reserve officer candidate with the Navy. Still working on that =)
PRK is waiverable. I know for the marines its the only accepted form of laser eye surgery due to it replacing the whole lens of the eye. In Lasik they cut a flap. For some reason Lasik is viewed as more problem prone even though the results are not out on this yet.
I would say with your PRK you should be good to go. You may need a waiver but it should not be difficult to obtain.